1. Field of Invention
The present invention relates to a wavelength division multiplex optical star coupler, a communication station, and an optical transmission system. In particular, the present invention relates to an optical transmission system that uses a wavelength division multiplex optical star coupler which combines an optical star coupler and a wavelength division multiplex optical coupler.
2. Description of Related Art
An optical fiber communication using an optical fiber as a light propagation line will be described as a conventional art. Usually, an optical star coupler of 1 to n branching is used for dividing an optical signal, which is generated from one source, into a plurality of signals. Here, n is an integer more than 2. It is possible to transmit the same optical signal to a plurality of multiple destinations by the optical star coupler. An optical fiber fusion type and an optical waveguide type are mainly used for the optical star coupler. The optical star coupler is not only used for distributing the light, but also used as a light multiplexer which multiplexes the optical signal output from different light sources because the optical star coupler usually works bidirectionally.
One of methods to use a single optical fiber efficiently is a wavelength division multiplex method that utilizes different wavelengths of the optical signals. Recently, a wavelength division multiplex optical coupler is used for multiplexing or de-multiplexing the optical signals having different wavelengths in the wavelength division multiplex method.
FIG. 1 shows a conventional wavelength division multiplex optical coupler. The wavelength division multiplex optical coupler 10 is basically the same as a 1 to 2 optical de-multiplexer or multiplexer. One side of the wavelength division multiplex optical coupler 10 is connected to a port P20, which can input and output the first optical signal λ1 and the second optical signal λ2. The other side of the wavelength division multiplex optical coupler 10 is connected to a port P22, which can input and output only the first optical signal λ1, and a port P24, which can input and output only the second optical signal λ2.
The first optical signal λ1 and the second optical signal λ2 have different wavelengths with each other. The first optical signal λ1 input from the port P20 is output to the port P22, and the second optical signal λ2 input from the port P20 is output to the port P24. The first optical signal λ1 input from the port P22 and the second optical signal λ2 input from the port P24 are output to the port P20. The ordinary wavelength division multiplex optical coupler is described in Japanese Patent Application Laid-Open No. H10-173265.
FIG. 2 shows a one-way direction wavelength division multiplex optical communication realized by the wavelength division multiplex method. One side of the wavelength division multiplex optical coupler 10 is connected to a port P26, which can input only the first optical signal λ1, and a port P28, which can input only the second optical signal λ2. The other side of the wavelength division multiplex optical coupler 10 is connected to the port P30, which can input the first optical signal λ1 and the second optical signal λ2. The first optical signal λ1 input from the port P26 and the second optical signal λ2 input from the port P28 are output to the port 30 through the wavelength division multiplex optical coupler 10. The direction of the communication is one-way from the port P26 and the port P28 to the port 30.
FIG. 3 shows a two-way direction wavelength division multiplex optical communication realized by the wavelength division multiplex method. One side of a wavelength division multiplex optical coupler 10a is connected to a port P32, which can input only the first optical signal λ1, and a port P34, which can output only the second optical signal λ2. The other side of the wavelength division multiplex optical coupler 10a is connected to a wavelength division multiplex optical coupler 10b. 
One side of the wavelength division multiplex optical coupler 10b is connected to a port P36, which can input only the second optical signal λ2, and a port P38 , which can output only the first optical signal λ1. The other side of the wavelength division multiplex optical coupler 10b is connected to the wavelength division multiplex optical coupler 10a. The wavelength division multiplex optical coupler 10a and 10b are connected by a single optical fiber.
The first optical signal λ1 input from the port P32 is output to the port P38 through the wavelength division multiplex optical couplers 10a and 10b. The second optical signal λ2 input from the port P36 is output to the port P34 through the wavelength division multiplex optical couplers 10a and 10b. The direction of the communication is bidirection from the port P32 to the port P38 and from the port P36 to the port P34.
Therefore, the optical communication that transmits an optical signal from one light source, such as a main unit, to a plurality of receiving terminals, such as sub units, and transmits the optical signals from the sub units to the main unit become possible by using the wavelength division multiplex optical coupler 10.
However, because the optical parts such as a star coupler and a wavelength division multiplex optical coupler use an optical fiber for the input/output, radiation loss will occur when bending the optical fiber. In the case of a quarts single mode fiber, the optical fiber has to be bent more than 30 mm of radius of curvature. Furthermore, there is the possibility of the breaking an optical fiber in the case of using the optical fiber strand with 250 μm of outward diameter to save the space for wiring. In the case of using a code with 2 mm to 3 mm of outward diameter to protect the optical fiber, the wiring space will increase because the outward diameter of the code is larger than the outward diameter of the optical fiber strand. Especially, as the number of branching of an optical star coupler is larger, the wiring space has to be larger. Therefore, the conventional art requires the work of packing the optical fiber without breaking it by considering the radius of curvature of the optical fiber.